In conventional marine propellers, cavitation occurs at the leading edge of the tip of the blade during high torque, high rpm and heavy load operation because high blade element angle of attack leads to separation of the boundary layer around the tip of the blade, which depresses local static pressures below that vaporizing water at ambient temperature, thereby creating vapor bubbles which collapse with destructive force to cause disturbance in the flow about the whole blade causing low propulsive efficiency and a very turbulent and energy absorbing wake. In the past, much has been done in the field of propeller design, particularly with respect to marine propellers, and in that field various forms and arrangements of ring propellers and shrouded propellers have been devised with the object of improving operational efficiency. In the case of shrouded propellers, the encircling structure is usually fixed or stationary, and in the case of ring propellers, the surrounding ring is fast on the propeller blade tips. None of these schemes has been particularly successful, however, and the present invention resulted from work directed to increasing propulsion efficiency through improved control of the fluid flow through the propeller system.